Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops onto an image substrate from a plurality of drop generators or inkjets, which are arranged in a printhead or a printhead assembly. For example, the printhead assembly and the image substrate are moved relative to one another and the inkjets are controlled to eject ink drops at appropriate times. The timing of the inkjet activation is performed by a printhead controller, which generates firing signals that selective activate inkjets to eject ink onto an image substrate. The image substrate may be an intermediate image member, such as a print drum or belt, from which the ink image is later transferred to a print medium, such as paper. The image substrate may also be a moving web of print medium or sheets of a print medium onto which the ink drops are directly ejected. The ink ejected from the inkjets may be liquid ink, such as aqueous, solvent, oil based, UV curable ink or the like, which is stored in containers installed in the printer. Alternatively, the ink may be loaded in a solid form and delivered to a melting device, which heats the solid ink to its melting temperature to generate liquid ink, which is supplied to a printhead.
During the operational life of an inkjet printer, inkjets in one or more of the printheads may become unable to eject ink in response to receiving a firing signal. The defective or inoperative condition of the inkjet may temporarily persist so the inkjet becomes operational after one or more image printing cycles. In other cases, the inkjet may remain unable to eject ink until a purge cycle is performed. A purge cycle may successfully unclog inkjets so that they are able to eject ink once again. Execution of a purge cycle, however, requires the imaging apparatus to be taken out of its image generating mode. Thus, purge cycles affect the throughput rate of an imaging apparatus and are preferably performed during downtime.
Compensation methods have been developed that enable an imaging apparatus to generate images even though one or more inkjets in the imaging apparatus are unable to eject ink. These compensation methods cooperate with image rendering methods to control the generation of firing signals for inkjets in a printhead. Rendering refers to the processes that receive input image data values and generate output image values. The output image values are used to generate firing signals, which cause the inkjets of a printhead to eject ink onto the recording media. Once the output image values are generated, a compensation method may use information regarding defective, also called inoperative, inkjets detected in a printhead to identify the output image data values that correspond to one or more defective inkjets in the printhead. The compensation method then finds a neighboring or nearby output image data value that can be adjusted to compensate for the defective inkjet. Preferably, an increase in the amount of ink ejected near the defective inkjet may be achieved by replacing a zero or nearly zero output image value with the output image value that corresponds to the defective inkjet.
Previously known compensation methods are useful for many situations, but some scenarios present issues regarding the camouflaging of missing inkjets, particularly in solid ink printers. One scenario involves the camouflaging of a missing inkjet that ejects black ink in a solid fill area. When a printer prints a solid fill area, almost all of the inkjets nearby the defective inkjet are ejecting ink into the area. Consequently, the firing signals to these inkjets cannot be modify to eject more ink into the area as most or all of the inkjets are at or near a maximum ink ejection amount. Additionally, placing more black ink adjacent an area devoid of black ink emphasizes the absence of the black ink rather than attenuating the absence. To address these problems, nearby inkjets in printheads that eject ink of a color other than black have been used to eject one or more inks of another color into the missing black ink gap. When this approach is used in a solid ink printer, however, the resulting gap is sometimes detectable. Solid inks are typically pressed against the media onto which the ink has been ejected by a spreader nip to present a more uniform appearance. When non-black inks are ejected into a gap in a black solid fill area, this spreading has been observed to restrict the spreading of black ink into the gap. Consequently, while the ink in the gap is not as noticeable as bare media, the ink in the gap still presents sufficient color contrast that the ink in the gap can be differentiated from the black ink on either side of the gap by an observer of the printed media. Therefore, compensation for missing black ink in a solid fill area, especially in a solid fill area produced by a solid ink printer, that presents a more uniform appearance would be useful.